Ammonia synthesis system

ABSTRACT

A system of apparatus and process for synthesizing ammonia are disclosed, which includes first and second catalytic synthesis converters with a heat exchanger interposed therebetween and operatively connected to the inlet and outlet of one converter and to the inlet of the other converter to permit a feed gas stream to be passed in heat exchange relationship with a partially synthesized gas stream passing from the outlet of the first converter to the inlet of the second converter. The converters and heat exchanger are mounted on a support platform, with one converter and heat exchanger mounted for movement with respect to the platform to accommodate dimensional changes caused by thermal expansion during operation.

United States Patent Wright et a]. 1March 20, 1973 [5 1 AMMONIASYNTHESIS SYSTEM 1,884,555 /1932 Brown .,l/l76 x 2,256,882 9/1941 Sebald"165/176 x inventors: Lee E. Wright, West CovinajAllan E. Pickford,Palos Verdes Estates, both of Calif.

Assignee: C. F. Braun & Co., Alhambra, Calif.

Filed: Feb. 8, 1971 Appl. No.: 113,333

[52] US. Cl ..23/289, 23/260, 23/288 H, 23/288 K, 23/288 R, /67, 165/83,

[51] Int. Cl. ..B01j 9/04, COlc l/04, F28d 7/06 [58] Field ofSearch.....23/289, 288 R, 288 K, 288 L, 23/288 M, 288 H, 260, 199, 198,l B; 248/2,

[56] References Cited UNITED STATES PATENTS 2,046,478 7/1936 O'Leary..23/289 X 2,475, l 09 7/1949 Pendleton l,869,236 7/l932 Baumann l65/l76X Primary Examiner-Joseph Scovronek Attorney-Lyon & Lyon 5 7 ABSTRACTconverter. The converters and heat exchanger are mounted on a supportplatform, with one converter and heat exchanger mounted for movementwith respect to the platform to accommodate dimensional changes causedby thermal expansion during operation.

5 Claims, 6 Drawing Figures AMMONIA SYNTHESIS SYSTEM BACKGROUND OF THEINVENTION This invention relates generally as indicated to the synthesisof ammonia and more particularly to a combination of apparatus andprocess in which such synthesis can advantageously be carried out.

The production of ammonia from natural gas, such as methane, of course,is well known. Generally, natural gas, water in the form of steam, andair are combined in a series of chemical reactions to produce ammonia ofa high degree of purity. The chemical reactions involved in theconventional ammonia process are:

Reforming Reaction CI-I H,O C 3H, Shift Reaction C0 H,0 C0, H, OxidationReaction 2H, O, 2H,O

Ammonia Synthesis 3H, N, ZNH,

The degree to which these reactions go to completion is generally afunction of temperature, pressure and constituents present in therespective reaction. While the reforming reaction is generally conductedat high temperatures and moderate pressure in the presence of a largeexcess of steam, the ammonia synthesis reaction is conventionallyconducted at very high pressures and relatively low to moderatetemperatures in order to obtain efficient conversion, inasmuch as theconversion reaction is enhanced by high pressure and low temperature andis greatly retarded by temperatures on the order of about 900-950 F.This is, of course, particularly significant since the ammonia synthesisreaction is exothermic. Moreover, from a practical standpoint, thesynthesis of ammonia is significantly limited not only by therequirement of a reactor capable of withstanding high pressure but alsoby the necessity for a minimum temperature of at least about 650-700 Fto initiate the reaction, which in turn necessitates careful andaccurate control of temperature.

As a result of such limiting factors, a relatively low percentage ofconversion is ordinarily obtained in a single pass through conventionalreactors. For the process to be economically feasible, however, theunconverted hydrogen and nitrogen must be recovered and subjected tofurther conversion, but before re-entering the reactor, the temperaturemust be reduced from about 950-1,100 F to about 650-700 F. In some priorsystems, this has been accomplished by passing the unconverted gasthrough one or more chillers and compressors and ultimately recycling tothe reactor. This has not resulted in a generally economical andacceptable system, since either an extremely large reactor or a veryrapid recycle rate is required to produce sufficient conversion.

In another previously used system, a single reactor is used whichincludes two somewhat separate reaction zones and temperature reductionmeans positioned generally between the reaction zones. Such system haspractical disadvantages due to the size of reactor required andinadequate and poorly controllable temperature reduction resulting fromthe heat exchange tubes being positioned adjacent the catalyst bed inwhich heat, of course, is generated by the exothermic reaction andimparted to the tubes.

SUMMARY OF THE INVENTION It is a principal object of the presentinvention therefore to provide a combination of apparatus and theaccompanying process for the synthesis of ammonia in which substantiallycomplete conversion may be obtained without encountering the aforenoteddisadvantages. In one form, the invention thus comprises a pair ofcatalytic synthesis converters with a heat exchanger positionedtherebetween and connected to the inlet and outlet of one converter andto the inlet of the other converter. The feed gas stream may thus bepassed in heat exchange relationship with a partially synthesized gasstream from the first converter. A support platform is also providedwith one of the converters fixedly mounted on the platform and the heatexchanger and other converter mounted thereon for movement with respectto the platform to accommodate dimensional changes caused by thermalexpansion during operation.

Another object of this invention is the provision of an ammoniasynthesis system in which the apparatus is arranged on a supportplatform in a relatively compact manner to achieve efficient utilizationof available plant facilities.

An additional object is the provision of a system in which effective andefficient temperature control is provided.

Yet another object of this invention is the provision of a system forsynthesizing ammonia in which efficient use is made of the heatgenerated during reaction by providing for heat exchange with thereaction product stream.

A further object of this invention is a provision of a system forsynthesizing ammonia in which means are included to compensate forthermal expansion during operation.

Other objects, features and advantages of this invention will beapparent to those skilled in the art after a reading of the followingmore detailed description.

DESCRIPTION OF PREFERRED EMBODIMENTS In the annexed drawings:

FIG. 1 is a top plan view of the ammonia synthesis system of thisinvention;

FIG. 2 is a side view of the system taken on line 2-2 of FIG. 1;

FIG. 3 is a perspective view of the apparatus with the platform omittedfor clarity of illustration;

FIG. 4 is an enlarged partial section view of one of the synthesisconverters;

FIG. 5 is an enlarged fragmentary view of the lower portion of theconverter of FIG. 4; and

FIG. 6 is an enlarged section view of one of the heat exchangers used inthe synthesis system.

Referring now to the drawings and more particularly FIG. 1 through 3inclusive, the synthesis system includes a pair of generally parallel,upright converters designated by numerals l and 2. A heat exchanger 3 isinterposed between and operatively connected to each of the converters.As shown perhaps most clearly in FIG. 3, the heat exchanger is connectedto converter 1 through pipes 4 and 5. Pipe 6 extends from the lowerportion of heat exchanger 3 and connects to the lower portion ofconverter 2. Pipe 7 is also connected to the lower portion of the heatexchanger and functions as an inlet for the incoming hydrogen andnitrogen which is admitted thereto prior to introduction into converter1.

In operation, a feed stream containing nitrogen and hydrogen isintroduced through pipe 7 into heat exchanger 3 and after heat exchangetherein is conveyed through pipe 5 to converter 1. Within converter 1synthesis of ammonia occurs by reaction under pressure between hydrogenand nitrogen, thereby producing a partially synthesized gas stream whichexits from converter 1 through pipe 4 and re-enters heat exchanger -3.Within the heat exchanger, the partially synthesized gas stream ispassed in heat exchange relationship with the incoming feed stream(which enters through pipe 7) and exits through pipe 6 through which itis conveyed to converter 2. Within converter 2, the gas stream undergoesfurther conversion, as will be described more completely hereinafter.

The system also includes a conventional steam generator 10 into whichthe effluent gas from converter 2 is taken through pipe 11. Within thesteam generator,

the gaseous ammonia stream is passed in heat exchange relationship withwater to produce steam for use in the reforming operation. The use ofsuch a generator is optional insofar as the present system is concerned,but since it is necessary to lower the temperature of the gaseousammonia for ultimate condensation to a liquid and also since steam isneeded for the reforming operation, it is obviously advantageous toutilize in this manner the heat which must be removed from the gaseousammonia.

After undergoing heat exchange in the steam generator 10, the effluentammonia gas is taken through pipe 12 to heat exchanger 13. The gastravels upwardly through the heat exchanger in heat exchangerelationship in the customary manner with the incoming gas stream whichhas been introduced through line 15 into heat exchanger 16. The incominghydrogen andnitrogen gas stream thus passes from heat exchanger 16 intoheat exchanger 13 and into heat exchanger 3 through line 7 as previouslydescribed. On the other hand, the ammonia gas exits from heat exchanger13 through line 18 and passes downwardly through heat exchanger 16,whereby its temperature is further lowered, and exits through line 19for further processing. For purposes of the present invention, suchfurther processing is not illustrated and will not be described indetail since it is conventional in the art, but would include furtherreduction in temperature by passing through heat exchangers and chillersand eventual condensation and separation as a substantially pure ammoniaproduct, with the unconverted'hydrogen and nitrogen being ultimatelymixed with fresh feed and recycled to the reactors.

The combination of apparatus is mounted on a platform which may be an 18inch to 3 foot thick concrete or steel platform, for example, and isthus provided in a relatively compact arrangement for efficientutilization of available facilities. As shown, the converters, heatexchangers and steam generators are all mounted in generally uprightposition and extend above and beneath platform 25. One of theconverters, in the embodiment illustrated, converter 2, is fixedlymounted on the platform, whereas converter 1 is mounted on the platform,as, for example through hanger rods or springs 26 so as to be capable ofboth lateral and vertical movement with respect to the platform toaccommodate dimensional changes caused by thermal expansion duringoperation. Heat exchanger 3 is likewise mounted on platform 25 by springmeans 27 and steam generator 10 and heat exchangers l3 and 16 aremounted by similar spring means 28, 29 and 30 respectively, to permitmovement to compensate for expansion and contraction during operation.At least one of the converters should be rigidly or fixedly mounted onthe platform to prevent uncontrolled movement of the apparatus and tocause the other apparatus to return to a normal position when the systemis not operating.

In FIGS. 4 and 5, the cross section of the catalytic converters is shownin greater detail. The converter there illustrated may be eitherconverter 1 or 2 and simply for the sake of illustration is designatedas converter 2. The internal components'of the converter, however, areessentially the same, even though as illustrated, converter 2 ispreferably considerably taller than converter 1 to provide a greaterresidence time for the feed stream to undergo reaction, therebyachieving a greater percent of equilibrium conversion to ammonia. Theconverter includes a shell 40 with a liner 41 positioned therein andspaced from the shell to provide an annular passageway 42 therebetween.Within the liner 41 is a catalyst bed 43 which contains a conventionalammonia synthesis catalyst such as iron or iron in combination withmolybdenum or other such substance to increase catalytic activity. Anoutlet screen 44 is provided adjacent the lower portion of the converterto prevent the solid catalyst particles from exiting with the gasstream. The outlet screen normally will be stainless steel or a nickelbased alloy and of a sufficiently small mesh to prevent the catalystparticles from passing therethrough. An inlet 45 is provided adjacentthe lower end of the converter through which the hydrogen and nitrogencontaining feed stream enters and passes upwardly through passageway 42and downwardly through catalyst bed 43, and ultimately exits throughoutlet 46.

As shown more clearly in FIG. 5, the inlet 45 includes a nozzle neck 50welded at 51 to shell 40. A layer of insulating material 52 such asceramic felt is provided adjacent outlet screen 44 enclosed by shroud 53of stainless steel or a nickel base alloy to prevent excessive transferof heat from the hot gases to the shell of the converter. The converter,of course, includes cover plates 55 and 56 connected by bolts 57 and 58to its top and bottom respectively.

Referring now to FIG. 6, the heat exchanger 3 is shown in greaterdetail. The design of heat exchanger 3 is illustrated and described morecompletely in the copending application of Allan Pickford, entitled HEATEXCHANGER and assigned to the assignee of this application, Ser. No.113,334, filed Feb. 8, 1971, and consequently will be described insomewhat abbreviated manner herein, although its inclusion in thepresent invention is of considerable importance. The incoming hydrogenand nitrogen feed stream enters the heat exchanger through opening 60and passes in and around tube bundle 61 which is positioned within theheat exchanger and spaced from shell 62. Although only two tubes areillustrated, the exchanger, of course,

includes the usual bundle of tubes. Transverse baffles 63 extend acrossthe interior of the heat exchanger as shown to cause the fluid to travelback and forth across the tube bundle as it passes through theexchanger, thereby providing for more efficient heat exchange. The fluidexits from the heat exchanger through outlet 65 from which it isconveyed through pipe 5 to reactor 1.

The heat exchanger 3 also includes a nozzle assembly 66 attached at itslower portion which supports the tube sheet 67 in which the tube bundleis retained whereby the tube sheet is detached from the shell of theheat exchanger. The tube sheet is mounted on channels 68 which areconnected to inlet 69 and outlet 70 of the nozzle assembly throughbellows 71 and horizontally extending channel 72 respectively. Apartition plate 75 is provided which extends between the tube sheet 67and plate 76 at the lower portion of the nozzle assembly to preventfluid entering nozzle 69 from exiting directly through outlet 70 withoutpassing through the tube bundle.

Pipe 4 is connected to inlet nozzle 69 of heat exchanger 3, while pipe 6is connected to outlet nozzle 70. The fluid passing within the tubebundle is thus the partially synthesized gas stream from reactor 1 whichthen exits through pipe 6 to reactor 2. A cover plate 78 is alsoprovided at the bottom of the exchanger connected to shell 62 by bolts79.

In a preferred form, the process of the present invention operatesessentially as follows. The hydrogen and nitrogen gas feed stream entersthrough line to heat exchanger 16 at a temperature of approximately 50to 200 F and under a pressure of approximately 1,450 to 4,500 psig, inwhich the temperature of the feed stream is increased to approximately150 F to about 350 F. The gas feed then passes under a pressure of fromabout 1,445 to about 4,475 psig to heat exchanger 13 in which thetemperature is again increased, to a temperature of about 400 F to about700 F. The exiting gas will be under a pressure of from approximately1,440 to about 4,450 psig and will be taken through line 7 to heatexchanger 3. The temperature of the hydrogen and nitrogen feed streamwill be increased within heat exchanger 3 to at least about 650 to 700F, which temperature is necessary to initiate reaction between hydrogenand nitrogen to produce ammonia. As mentioned previously, thetemperature of the feed stream entering converter 1 will be maintainedno greater than about 800 F., as the conversion reaction underequilibrium conditions is not favored by higher temperature. Thepressure of the incoming gas stream to reactor 1 will be from about1,410 to about 4,425 psig. Within reactor 1, the temperature of the gasstream increases since the conversion reaction is exothermic. Thetemperature of the product gas which passes from reactor 1 through pipe4 to heat exchanger 3 will be on the order of about 800 to 1,100 F andat a pressure of about 1,385 to about 4,410 psig. Within the heatexchanger 3, the temperature is lowered as previously described so thatthe temperature of the gas stream in line 6 prior to entering reactor 2will be on the order of from about 650 to about 800 F. The pressure willbe about 1,350 to about 4,385 psig.

Within reactor 2, further reaction of the unconverted hydrogen andammonia occurs. In reactor I, normally about 30 to about mole percent,based on the percent of hydrogen in the fresh feed, of the incominghydrogen and nitrogen is converted with from about 70 to about 10 molepercent occurring in reactor 2. Since the reaction between hydrogen andnitrogen in reactor 2 is also exothermic, the temperature of the gasincreases to around 700 to about 1,000 F. The pressure is from about1,320 to about 4,365 psig.

In steam generator 10, the temperature of the gas is reduced asexplained previously to a temperature on the order of about 600 to about660 F. The pressure of the gas exiting from the steam generator isnormally from about 1,260 to about 4,340 psig. After passing throughheat exchangers 13 and 16, the temperature of the ammonia gas has beenreduced to about to about 250 F, and the pressure is about 1,250 toabout 4,325 psig.

It will be appreciated from the foregoing description that the presentinvention provides a system for synthesizing ammonia in which thetemperature can be properly controlled to insure efficient reaction. Bythe same token, the present invention provides a relatively simple andcompact arrangement of apparatus and compensates for thermal movementand thus represents a significant improvement over previously knownconventional systems.

We claim:

1. A system of apparatus for synthesizing ammonia from a mixture ofnitrogen and hydrogen in a feed gas stream, comprising in combination:first and second parallel upright synthesis converters, each having aninlet and outlet and each containing a catalyst bed operativelypositioned between the inlet and the outlet, an upright elongated heatexchanger physically positioned between said converters, means includinga pipe for delivering a feed gas stream to said heat exchanger, pipingconnecting said heat exchanger to the inlet and outlet of the first ofsaid converters and to the inlet of said second converter, whereby thefeed gas stream may be passed in heat exchange relationship with apartially synthesized gas stream passing from the outlet of said firstconverter to the inlet of said second converter, a support platform,means fixedly supporting said second converter on said platform toprevent uncontrolled movement of such system of apparatus, and meansmounting said heat exchanger and said first converter on said platformfor lateral and vertical movement with respect thereto, to accommodatedimensional changes occasioned by thermal expansion of said piping.

2. The apparatus of claim 1 in which means are provided for heatexchanging effluent gas from the outlet of said second converter withfeed gas upstream from said pipe, the latter said means including asecond and third upright heat exchanger mounted on said platform forthermal expansion movement with respect thereto.

3. A system of apparatus for synthesizing ammonia from a mixture ofnitrogen and hydrogen in a feed gas stream, comprising in combination:first and second parallel upright synthesis converters each having aninlet and an outlet and each containing a catalyst bed operativelypositioned between the inlet and the outlet, an upright elongated heatexchanger physically positioned between said converters, means includinga first pipe for delivering a feed gas stream to said heat exchanger,second, third and fourth pipes connecting said heat exchanger to theinlet and outlet of the first of said converters and to the inlet ofsaid second converter respectively, whereby a feed gas stream may bepassed in heat exchange relationship with a partially synthesized gasstream passing from the outlet of said first converter to the inlet ofsaid second converter, a support platform, said heat exchanger andconverters extending both above and beneath said platform, means fixedlysupporting said second converter on said platform to preventuncontrolled movement of such system of apparatus, and means suspendingsaid heat exchanger and said first converter from said platform forlateral and vertical movement with respect thereto, to accommodatedimensional changes occasioned by thermal expansion of said pipes.

4. The apparatus of claim 3 in which means are provided for heatexchanging effluent gas from the outlet of said second converter withfeed gas upstream from said first pipe, the latter said means includingsecond and third upright heat exchangers suspended from said platformfor thermal expansion movement with respect thereto. 7

5. The system of apparatus of claim 1 in which said heat exchangerincludes an outlet connected to said first converter and a nozzleassembly at the lower end thereof, said nozzle assembly including afirst inlet nozzle communicating with said pipe means for the deliveryof feed gas to said heat exchanger, whereby feed gas may be passedthrough said heat exchanger and said outlet of said heat exchanger tosaid first converter, a second inlet nozzle communicating with saidfirst converter, and an outlet nozzle connected to said secondconverter, thus permitting partially synthesized gas from said firstconverter to pass through said heat exchanger and to said secondconverter.

t i t t UNITED STATES PATENT AND TRADEMARK OFFICE Certificate Patent No.3,721,532 Patented March 20, 1973 Lee E. Wright and Allan E. PickfordApplication having been made by Lee E. Wright and Allan E. Pickford, theinventors named in the patent above identified, and C. F. Braun &Company, Alhambra, California, a corporation of California, theassignee, for the issuance of a certificate under the provisions ofTitle 35, Section 256, of the United States Code, adding the name ofWayne A. Glover as a joint inventor, and a showing and proof of factssatisfying the requirements of the said section having been submitted,it is this 3rd day of May 1977, certified that the name of the saidWayne A. Glover is hereby added to the said patent as a joint inventorwith the said Lee E. Wright and Allan E. Pickford.

FRED W. SHERLING,

Associate Solicitor.

2. The apparatus of claim 1 in which means are provided for heatexchanging effluent gas from the outlet of said second converter withfeed gas upstream from said pipe, the latter said means including asecond and third upright heat exchanger mounted on said platform forthermal expansion movement with respect thereto.
 3. A system ofapparatus for synthesizing ammonia from a mixture of nitrogen andhydrogen in a feed gas stream, comprising in combination: first andsecond parallel upright synthesis converters each having an inlet and anoutlet and each containing a catalyst bed operatively positioned betweenthe inlet and the outlet, an upright elongated heat exchanger physicallypositioned between said converters, means including a first pipe fordelivering a feed gas stream to said heat exchanger, second, third andfourth pipes connecting said heat exchanger to the inlet and outlet ofthe first of said converters and to the inlet of said second converterrespectively, whereby a feed gas stream may be passed in heat exchangerelationship with a partially synthesized gas stream passing from theoutlet of said first converter to the inlet of said second converter, asupport platform, said heat exchanger and converters extending bothabove and beneath said platform, means fixedly supporting said secondconverter on said platform to prevent uncontrolled movement of suchsystem of apparatus, and means suspending said heat exchanger and saidfirst converter from said platform for lateral and vertical movementwith respect thereto, to accommodate dimensional changes occasioned bythermal expansion of said pipes.
 4. The apparatus of claim 3 in whichmeans are provided for heat exchanging effluent gas from the outlet ofsaid second converter with feed gas upstream from said first pipe, thelatter said means including second and third upright heat exchangerssuspended from said platform for thermal expansion movement with respectthereto.
 5. The system of apparatus of claim 1 in which said heatexchanger includes an outlet connected to said first converter and anozzle assembly at the lower end thereof, said nozzle assembly includinga first inlet nozzle communicating with said pipe means for the deliveryof feed gas to said heat exchanger, whereby feed gas may be passedthrough said heat exchanger and said outlet of said heat exchanger tosaid first converter, a second inlet nozzle communicating with saidfirst converter, and an outlet nozzle connected to said secondconverter, thus permitting partially synthesized gas from said firstconverter to pass through said heat exchanger and to said secondconverter.